The conventional apparatus for MRI comprises a magnet producing a strong DC magnetic field Bo in the imaged object. Typically the magnet is a superconducting magnet and its axis may either be oriented vertically or horizontally, depending upon the design. Within a bore is a set of gradient coils producing a distribution of magnetic fields within the object being imaged as is required for MRI. One or more RF coils are placed near the sample to produce an RF magnetic field B1 to stimulate the magnetic resonance of a given nuclear species in the object, and the same coil or a separate coil to detect any resonances that are produced in the object. The apparatus includes a RF transmitter to generate the required RF current supplied to the stimulating RF coil and a RF receiver to amplify and detect the response of the stimulated nuclei. A controller is used to control the field gradients, the RF transmitting signal the receiver response, and to collect the data and form the required image.
When a single RF coil is used, the same coil serves to produce the stimulating RF field B1, and to receive the response from the nuclei. The RF coil may be a volume coil that surrounds the sample, such as a solenoid coil, a Helmholtz coil pair, a birdcage coil, or a surface coil that only partially surrounds the sample. The RF coil is switched to the RF transmitter that generates an RF pulse that, when coupled to the RF coil, stimulates the nuclei. The RF coil is then switched to the receiver that amplifies and detects the responding signal from the nuclei. Radio frequency transmission lines couple the RF coil or coils located in the magnet with the MRI apparatus located nearby outside of the magnet.
In systems with two RF coils, one is used to produce the stimulating RF field B1 and the other to receive the response from the nuclei. These systems typically use a large volume coil to produce a uniform RF magnetic field over the sample volume, and a surface coil to pick up signals in the nearby region of the sample with high sensitivity. The transmitter is first pulsed on for a short time period, after which the receiver is turned on to detect the nuclear response signal. When the transmitter is on the receiver channel is partially blocked to prevent the strong transmitter signals from damaging the receiver. The problem that occurs in these systems arises from the coupling between the two coils. During transmit phase, RF fields from the transmitter coil induce voltages in the receiver coil, causing currents to flow in the receiver coil that produce additional RF fields in the object being imaged. These fields add and subtract from the RF fields of the transmitter coil causing the resulting RF field to be inhomogeneous. Also during the receive phase, signals from the nuclei are absorbed in the transmitter coil with a loss of signal power. In addition, any noise in the transmitter circuit is coupled into the receiver thereby lowering the signal to noise ratio. One way to alleviate this problem is to place the coils such that their RF magnetic fields are orthogonal. This in practice is difficult to do with a high degree of accuracy, making it desirable to find other ways to reduce these undesirable effects.
A number of authors (for example, A. Asfour et al. “Instrumentation and Measurement Technology Conference Proceedings”, 2008-05-12, pages 945-950; U.S. Pat. No. 5,559,434) have shown that by actively detuning the receiver coil during the transmitter phase and actively detuning the transmitter coil during the receiver phase greatly reduces these problems. Active detuning was achieved by using a pin diode to switch in an inductive or capacitive element across some part of the transmitter or receiver coil. The switch is activated by passing or removing an externally applied bias current flowing through the pin diode as illustrated in FIG. 3 of the Asfour, et al. reference. This has been found to work, however it requires fairly large externally applied bias voltages to completely turn on or off a pin diode when large RF voltages are present. To switch off the diode the externally applied bias voltage must be sufficiently large so that in no part of the RF cycle will the RF voltage overcomes the bias voltage and momentarily turns on the diode.
In the U.S. Pat. No. 7,501,828 it is suggested to devise a circuit that employs two pin diodes that does not require any bias voltage when the detune circuit is not active. This patent describes a circuit wherein a pin diode is inserted in each lead of the reactive element used to provide the detuning. Either both cathodes or both anodes of the two pin diodes connect separate leads of the reactive element. When activated by an external voltage, both pin diodes are turned on and the reactive element is coupled to the coil and provides the detuning. When no external voltage is applied one of the diodes is always off since the RF voltage across the diodes always has the opposite polarity. The external bias voltage source is decoupled from the RF coil by radio frequency choke coils that have high impedance at RF frequencies but low impedance to the switching voltages.
A RF receiver with a surface RF coil is particularly applicable to the imaging of small animals. To observe the magnetic resonance signals with the highest sensitivity the coil structure must be placed very close or perhaps in contact with the region of the object or animal being studied can provide the high filling factor and high sensitivity. The same RF coil or a different RF coil may serve as the transmitter coil.
One variation of a surface coil is described in U.S. Pat. No. 5,898,306. This patent describes a surface coil in the form of two coupled ladder resonator coils, with a first mode circuit path for detecting or generating magnetic flux in a vertical axis from the surface of the coil and a second mode circuit path for detecting or generating magnetic flux in a parallel to the surface of the coil, with the first mode and second mode currents 90 degrees out of phase. The design features fixed capacitors in the resonator rungs with the rungs coupled together by inductors forming a low-pass design.
U.S. Pat. No. 6,169,401 describe a flexible open quadrature high-pass ladder structure RF surface coil in magnetic resonance imaging. This design includes a central rung having a capacitive element Cv disposed symmetrically about a midpoint, and a like number of additional rungs are disposed parallel to and symmetrically on opposite sides of the central rung. The side elements include fixed capacitive elements Ca, which interconnect adjacent ends of each of the rungs forming a high-pass circuit. Both of these patents require fixed capacitors either in series with the rung elements or between rung elements, which because of their size, cause problems particularly in surface coils for the imaging of small animals. In addition fixed capacitors typically have a different magnetic susceptibility that the surrounding region thereby causing unwanted gradients in the DC magnetic field Bo.